1. Field of the Invention
The present invention refers to an optical device, preferably realised in Planar Lightwave Circuits or PLC technology, for simultaneously generating and processing optical codes, which allows the label generation and processing to be executed directly in the optical domain and which is accurate, reliable, simple, and inexpensive. In particular, the device may be used in Multi Protocol Label Switching or MPLS communication networks, and in Code Division Multiple Access or CDMA network.
1. Description of the Related Art
The presently most diffused communication networks, employing the IP (Internet Protocol) protocol, use a SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) transport layer over which an ATM (Asynchronous Transfer Mode) switching layer rests, on which data according to IP protocol travel. In particular, voice traffic typically travels on SONET/SDH layer. Such networks allows even more than one wavelength to be employed for transmission, according to WDM (Wavelenght Division Multiplexing). This four-layer architecture is too slow for managing high traffic volume at reasonable costs. But above all it is inefficient as far as scalability and flexibility of the whole network are concerned.
For these reasons, MPLS system has been proposed and standardised by IETF (Internet Engineering Task Force) organisation, system which is based upon a set of protocols used for scaling and managing optical networks so as to reduce the protocol stack, by incorporating the SONET/SDH and ATM layers into one sole IP/MPLS layer. In fact, as described by K. H. Liu in “IP Over WDM”, John Wileym & Sons, Ltd West Sussex, England 2003, by M. Murata and K. I. Kitayama in “A perspective on photonic multiprotocol label switching”, IEEE Network, July/August, pp. 56-63, 2001, by R. Xu, Q. Gong and P. Ye in “A novel IP with MPLS over WDM-based broad-band wavelength switched IP network”, IEEE J. Lightwave Technol., vol. 19, n. 5, pp. 596-602, 2001, by M. Koga in “Photonic MPLS router”, Proc. Lasers and Electro-Optics, (CLEO), Long Beach, Calif. USA, vol. 1, pp. 581-582, vol. 1, 2002, and by D. J. Blumenthal in “Photonic packet and all-optical label switching technologies and techniques”, Opt. Fiber Comm. Conf. (OFC), Anaheim, Calif. USA, paper W03, pp. 282-284, 2002, MPLS protocols overlap IP protocols for simplifying traffic engineering and allowing the use of the network resources to be efficient.
With reference to FIG. 1a, at input node 1 of a MPLS network, a label 2 having constant format is inserted at the head (or at the tail) of each data packet 3. In particular, such labels are codes generally having up to 32 bits, each label bit being more properly called chip. At each following node 4, the packet 3 is routed on the basis of the value of the label 2 itself until destination node 5, which finally receives the packet 3.
In other words, MPLS networks generates virtual link or tunnel connecting external nodes 1 and 5 of an optical network. If a data packet 3 is entered in the input of a tunnel, the normal IP procedure are suspended and packets are routed towards destination node 5 on the basis of only the value of the labels 2, according to the so-called label switching.
Hence, MPLS protocol does not replace the normal routing of IP packets, but it overlaps this protocol for increasing data transmission speed, allocating a sufficient band for traffic flows with different QoS (Quality of Service) requirements.
However MPLS networks present some drawbacks.
The main limitation in current MPLS networks is the fact that label generation and processing occur at electronic level instead of optical, limiting in a very large measure the maximum transmission speed, which is reduced to the order of 10 Gbit/sec.
In fact, current technology uses labels, i.e. codes with length up to 32 chips which are inserted at the front (or at the back) of the data packet; the so obtained electric signal is converted in an optical signal and transmitted into the MPLS network. At each single node, the optical signal, comprising the data packet 3 and the label 2, has to be reconverted in an electric signal from which the label 2 is extracted. The label 2 is read by carrying out the correlation between the label 2 itself and all the other labels, which are inserted into a table of stored codes.
The labels are all orthogonal with respect to each other and only when the input label matches its corresponding one in the table there is a peak of the auto-correlation function. In such case, the data packet 3 provided with the same label 2 is converted again from electric to optical signal and routed to the subsequent node. In the case when it is necessary a label change, the data packet is provided with a new label, through a label swapper, and then routed. All the above involves that at each node an electric-optical-electric double conversion is needed, using a photodetector and a laser source. It is known that these devices make up more than 75% of the cost of an optical network and that hence it is financially advantageous to reduce their use at most.
Some solutions to these drawbacks have been proposed by K.-I. Kitayama, N. Wada, and H. Sotobayashi in “Architectural considerations for photonic IP router based upon optical code correlation”, IEEE J. Lightwave Technol., vol. 18, n. 12, pp. 1834-1844, 2000, by K.-I. Kitayama and N. Wada in “Photonic IP routing”, IEEE J. Lightwave Technol., vol. 11, n. 12, pp. 1689-1691, 1999, by N. Wada and K.-I. Kitayama in “Photonic IP routing using optical codes: 10 Gbit/s optical packet transfer experiment”, Proc. Optical Fiber Communication Conference (OFC), Baltimore, Md. USA, vol. 2, paper WM51-1, pp. 362-364, 2000, and by K.-I. Kitayama and M. Murata in “Photonic access node using optical-code based label processing and its applications to optical data networking”, IEEE J. Lightwave Technol., vol. 19, n. 10, pp. 1401-1415, 2001. In particular, in such architectures it has been proposed to generate and process labels directly in the optical domain.
Since in order to read a label it is necessary to carry out N correlations between the input label and all the N labels of the table, such solutions carry out said N correlations in the optical domain by using N different devices, one for each label.
This consequently entails other drawbacks, due to the complexity and to the cost of the correlation apparatus, which requires N copies of each packet and N correlators.
Another solution for managing MPLS networks directly in the optical domain is to use multi protocol wavelength switching or MPλS (Multi Protocol Lambda Switching) systems, also called generalised MPLS systems, wherein different wavelengths are used as labels.
However, also these systems present some drawbacks.
The main limitation of these systems is the small code cardinality, i.e. the small number of labels, due to the strict one-to-one correspondence between labels and corresponding wavelengths λ.
Moreover, generalised MPLS systems need tunable laser sources for generating different wavelengths which are more expensive than normal laser sources.
Furthermore, these systems need a demultiplexer at each node for reading the different labels.
What has been described so far is valid also for CDMA networks, wherein multiple access techniques make simultaneous access to a transmission channel possible for a large number of users.
In particular, the CDMA technique assigns a specific code to each user, code which is independent of the information signal to be transmitted. The encoding operation, called spreading, consists of multiplying the code assigned to each single user by the information signal. Instead, in the decoding operation, the receiver carries out a correlation between the received signal and the code of the user which is intended to be received (despreading). In order to avoid interference among the various users simultaneously accessing to the network, it is necessary that the codes are orthogonal with respect to each other.
As described by D. D. Sampson, G. J. Pendock, and R. A. Griffin in “Photonic Code-division multiple access communications” Fibre and lnt. Opt., vol. 16, pp. 129-157, 1997, by M. Azizoglu, J. A. Salehi, and Y. Li, in “Optical CDMA via temporal codes” IEEE Trans. Commun., vol 40, n. 7, pp. 1162-1170 (1992), by J. A. Salehi in “Code division multiple-access techniques in optical fiber networks-part I: fundamental principles,” IEEE Trans. Commun., vol 37, n. 8, pp. 824-833 (1989), by M. E. Marhic, in “Coherent optical CDMA networks”, J. Lightwave Technol., vol. 11, n. 5/6, pp. 854-864 (1993) and by K.-I. Kitayama in “Code division multiplexing lightwave networks based upon optical code conversion” IEEE J. Select. Areas Commun., vol. 16, n. 7, pp. 1309-1319, 1998, in a multiple access optical network, as the one schematically shown in FIG. 1b, the signals transmitted by all the users 50 are distributed to each receiver 51 by means of a star coupler 52. If data coding and decoding are carried out in the optical domain, aggregated transmission speeds very much higher than the ones possible with electronic encoders and decoders are reached. In all the architectures proposed in literature N different encoders 53, one for each user 50, are used. At reception, decoding is carried out by using an adapted filter, once that the desired user code is known. Obviously it is necessary to have N different decoders 54, one for each code.
MPLS and CDMA networks present further drawbacks.
In fact, in order to precisely distinguish the different optical codes, it is necessary that the peak of the auto-correlation function is as higher as possible whereas the cross-correlation function must be close to zero everywhere. A review of characteristics and properties of the optical codes proposed in literature has been made by S. W. Lee and D. H. Green in “Coding for coherent optical CDMA networks”, IEEE Proc. Commun., vol. 145, n. 3, pp. 117-125, 1998, by S. W. Lee and D. H. Green in “Performance analysis method for optical codes in coherent optical CDMA networks”, IEEE Proc. Commun., vol. 147, n. 1, pp. 41-46, 2000, by S. W. Lee and D. H. Green in “Performance analysis of optical orthogonal codes in CDMA LANs”, IEEE Proc. Commun., vol. 147, n. 4, pp. 256-271, 1998, by F. R. K. Chung, J. A. Salehi, and V. K. Wei in “Optical orthogonal codes: design, analysis, and applications” IEEE Trans. Inform. Theory, vol. 35, n. 3, pp. 595-604, 1989, and by G.-C. Yang and T. E. Puja in “Optical orthogonal codes with unequal auto- and cross-correlation constraints”, IEEE Trans. Inform. Theory, vol. 41, n. 1, pp. 96-106, 1995. The codes proposed by K.-I. Kitayama, N. Wada, and H. Sotobayashi in “Architectural considerations for photonic IP router based upon optical code correlation”, IEEE J. Lightwave Technol., vol. 18, n. 12, pp. 1834-1844, 2000, and by K.-I. Kitayama in “Code division multiplexing lightwave networks based upon optical code conversion” IEEE J. Select. Areas Commun., vol. 16, n. 7, pp. 1309-1319, 1998 are the Hadamard codes which present an auto correlation peak or ACP equal to ACP=N2, whereas the maximum value of the cross-correlation function or CCP (Cross Correlation Peak) is CCP=(N−1)2. By way of example, in the case when N=8, the auto correlation peak is equal to ACP=64, whereas the maximum value of the cross-correlation function is CCP=49. Consequently, the parameter of code orthogonality is quite high, equal to r=CCP/ACP=49/64=0.77, thus not enabling particularly accurate performances for routers of a MPLS network and for detection in CDMA systems.